Integrated chemical process

ABSTRACT

A mineral carbonation process, characterized in that the silicate feedstock is thermally activated by using heat generated from the combustion of fuel prior to reacting the activated slurry feedstock with carbon dioxide.

The present invention relates to a process for the permanent and safesequestration of carbon dioxide gas and is particularly concerned withan efficient integrated process for the chemical conversion of carbondioxide to solid carbonates thereby reducing the accumulation of carbondioxide in the atmosphere.

The sequestration of carbon dioxide gas in repositories that areisolated from the atmosphere is a developing technology that is widelyrecognised as an essential element in global attempts to reduce carbondioxide emissions to the atmosphere. The rapid increase in atmosphericcarbon dioxide concentrations is of concern due to its properties asgreenhouse gas and its contribution to the phenomena of global warmingand climate change. Prototype demonstration facilities for the captureand sequestration of carbon dioxide exist in several countries. Whilevarious technologies exist for the capture and concentration of carbondioxide in combustion flue gases, most current facilities utiliseunderground sequestration known as geosequestration. This may occur indepleted oil or gas reservoirs or other underground porous formationsthat are suitably isolated from the atmosphere. These reservoirs orformations may be situated under land or sea. Another possiblesubterranean repository for carbon dioxide gas are so-called salineaquifers. Direct storage of carbon dioxide in the deep ocean has alsobeen investigated but has yet to be successfully demonstrated on anysignificant scale.

Another field of study is that known as mineral carbonation; wherebycarbon dioxide is chemically reacted with alkaline and alkaline-earthmetal oxide or silicate minerals to form stable solid carbonates. Theuse of this route in a mineral carbonation process plant using mineralsthat have been extracted and processed is known as ex-situ mineralcarbonation, as opposed to in-situ carbonation whereby carbon dioxide isdeposited into underground mineral formations and reacts over longertimeframes with such minerals in existing underground formations. Thepresent invention is concerned with the ex-situ approach to carbondioxide sequestration via mineral carbonation.

The invention assumes that a stream containing carbon dioxide isavailable for such mineral carbonation. Such streams may originate fromflue gases from any combustion process, or from processes known in theart as gasification or gas reforming, as well as from typical chemicalmanufacturing processes such as ammonia or Portland cement manufacture.The concentration of carbon dioxide in such streams may be substantiallyraised via technological routes known in the field. These includeso-called carbon capture technologies such as those employing membraneseparation technology or alternatively those employing carbon dioxidesolvents such as amines. In the latter case, these solvents capture thecarbon dioxide from dilute streams such as flue gases and then undergosolvent regeneration to release the concentrated streams of carbondioxide and the regenerated solvent for use in further capture.Alternatively, in a process known as “oxy-fuel combustion”, streams ofconcentrated carbon dioxide and water vapour may be formed directly inthe combustion processes via the use of oxygen rather than air to feedthe combustion process. Another process known as gasification produceshydrogen and relatively pure carbon dioxide streams through thegasification of hydrocarbonaceous fuels under suitable processconditions.

The present invention is concerned with the solidification of suchstreams of carbon dioxide in the process of mineral carbonation asdescribed herein. While it is advantageous to use such highlyconcentrated streams of carbon dioxide in the present invention, the useof lower purity streams is not precluded. In particular, the presence ofwater in such streams is not necessarily unfavourable since the processuses aqueous slurries whose water content may be readily adjusted ifrequired. Furthermore, the key aspects of the current invention may beapplied to slower or less intensive processes for carbon dioxidesequestration. These may include for example carbon dioxidesequestration from the atmosphere. The present invention provides theappropriate integrated activation process for the alkali or alkali earthmetal silicate feedstocks and the necessary integrated solvent processesfor the carbonation reactions required for viable ex-situ sequestration.

By way of example only, the following reviews and papers describe thesevarious sequestration technologies and their status:

-   Metz, B., Davidson, O., De Coninck H., Loos. M and Meyer, L.    (eds), 2006. Carbon Dioxide Capture and Storage—IPCC Special Report,    UN Intergovernmental Panel on Climate Change, ISBN92-9169-119-4.-   Herzog, H., 2002. Carbon Sequestration via Mineral Carbonation:    Overview and Assessment (available on the MIT University website for    Carbon Capture & Sequestration Technologies).-   Huijgen, W. J. J. and Comans, R. N. J., 2005. Carbon dioxide    sequestration by mineral carbonation—Literature Review Update    2003-2004, ECN-C-05-022.-   Lackner, K. S., Grimes, P. and Ziock, H-J., 2001. Capturing Carbon    Dioxide From Air, 1^(st) National Conference on Carbon Sequestration    May 14-17, 2001, USA (available on the website for the National    Energy Technology Laboratory, Department of Energy, USA).-   O'Connor, W. K., Dahlin, D. C., Rush, G. E., Gerdemann, S. J.,    Penner, L. R. and Nilsen, D. N., 2005. Aqueous Mineral    Carbonation—Mineral Availability, Pre-treatment, Reaction    Parametrics and Process Studies—Final Report, DOE/ARC-TR-04-002,    Albany Research Center, US DOE.-   ZECA Corporation, 2006. Overview-carbon dioxide.

Furthermore, some examples of related prior art in the patent literaturemade reference to here are listed below:

-   United States Patent Application US 2001/0022952 A1 by Rau and    Caldeira, Method and Apparatus for Extracting and Sequestering    Carbon Dioxide.-   United States Patent Application US 2004/0131531 A1, Geerlings,    Mesters and Oosterbeek, Process for Mineral Carbonation with Carbon    Dioxide.-   United States Patent Application No. 2004/0126293 A1 by Geerlings    and Wesker, Process for Removal of Carbon Dioxide from Flue Gases.-   United States Patent Application US 2004/0213705 A1 by Blencoe,    Palmer, Anovitz and Beard, Carbonation of Metal Silicates for    long-term CO₂ Sequestration.-   US Patent Application No. 2004/0219090 A1 by Dziedic, Gross, Gorski    and Johnson, Sequestration of Carbon Dioxide.-   United States Patent Application No. US 2005/0180910 A1 by Park and    Fan, Carbon Dioxide Sequestration using Alkaline Earth Metal-Bearing    Minerals.

Mineral carbonation has a number of potential advantages over othermethods of carbon dioxide sequestration. These include relativepermanence and stability and the elimination of any risks of leakage ofcarbon dioxide gas. Furthermore, suitable subterranean sites forgeosequestration do not exist at all locations where they are required.The chemical reactions of mineral carbonation are also thermodynamicallyfavoured, with an exothermic release of energy due to the formation ofthe carbonates. Many of the minerals required for mineral carbonationare abundant and widely distributed globally. These minerals may bereadily mined and subjected to known comminution and other technologies.They are generally benign and the environmental and safety risks arereadily manageable. In particular, the mineral broadly known asserpentine has been estimated to be available in quantities sufficientto sequester all global emissions of carbon dioxide from known fossilfuel reserves.

Examples of mineral carbonation chemical reactions are given here:½Mg₂SiO₄+CO₂═MgCO₃+½SiO₂CaSiO₃+CO₂+2H₂O═CaCO₃+H₄SiO₄⅓Mg₃Si₂O₅(OH)₄+CO₂═MgCO₃+⅔SiO₂+⅔H₂O

The latter example is that of serpentine, which is a favourablefeedstock due to its relative abundance. Much attention has beenfocussed on serpentine for that reason.

However, to date mineral carbonation is still only recognised as beingin the research phase with no viable industrial processes beingreported. The review by Metz et al. (2006) to the United NationsIntergovernmental Panel on Climate Change concludes that the energyrequired for carbonation would be in the range 30-50% of the energyoutput of the associated coal-fired power plant, rendering mineralcarbonation unviable. They note that research efforts are directed atfinding routes to increase the reaction rates and make the carbonationprocess more energy efficient. Leading researchers in this field fromthe Albany Research Center (O'Connor et al., 2005), similarly concludedin their final report on aqueous mineral carbonation that the cost ofcarbon dioxide sequestration via mineral carbonation would be in therange US$ 54-199 per tonne of carbon dioxide. They conclude that olivineand wollastonite exhibit the best potential for utilisation inindustrial process and dismiss serpentine as completely unviable due tothe high energy input required for activation of serpentine. They do notteach any means of achieving such a viable activation and theircalculations are based on the use of electrical energy for activation ofserpentine. They conclude that the use of serpentine in ex-situindustrial processes can be ruled out and label it as an impracticalmethodology. They conclude further that the only likely application ofserpentine in sequestration is as a slowly reactive matrix for in-situgeosequestration of carbon dioxide.

Various researchers have continued to explore methods of improving thereactivity of serpentine and other alkali metal or alkaline earth metalcontaining minerals. For example, US Patent Application No. US2005/0180910 A1 by Park and Fan presents a process that alters the pH ofthe mineral suspension and utilises a fluidised bed reactor withinternal grinding media to activate the serpentine. Their inventionrelates to the dissolution of magnesium-containing minerals in weakacids assisted by physical surface activation and subsequent increasingof the pH of the solution after contact with carbon dioxide toprecipitate the carbonates and sulphates. Their methodology has beendescribed in US Patent Application No. US 2005/0180910 A1. The inventionof Park and Fan does not teach any thermal activation of themagnesium-containing mineral by any means nor several of the associatedprocess improvements or applications of the present invention.

ZECA Corporation (2006) has published information on a process tosequester carbon dioxide emissions from coal-fired electricitygeneration using mineral carbonation of magnesium silicate minerals.However, no direct teaching of a viable process to achieve this isgiven, although reference is made to a patent-pending process based onthe work of the Albany Research Center. As noted herein however,published work from the workers at Albany Research Center has ruled outthe use of serpentine in ex-situ mineral sequestration of carbon dioxideand has not taught a means of achieving a viable process.

Other prior art teaches other methods and technologies that do notanticipate the current invention. For example, US Patent Application No.2004/0126293 A1 by Geerlings and Wesker reports on a process thatutilises the heat release from a mineral carbonation reaction to provideheat for the regeneration of solvent used in carbon dioxide captureprocesses from flue gases. No teaching is made in relation to themineral carbonation process itself.

In another US Patent Application No. 2004/0131531 A1, Geerlings et al.describe a process for mineral carbonation wherein carbon dioxide isreacted with a bivalent alkaline earth metal silicate which is immersedin an aqueous electrolyte solution. It is noted that such disclosure ofan electrolyte salt had been made earlier by O'Connor et al. (2001). Nomention is made in US Patent Application No. 2004/0131531 A1 in relationto activation of such bivalent alkaline earth metal silicate. Theexamples given in said application by Geerlings et al. are limited towollastonite and no activation is required for the carbonation reaction.

US Patent Application No. 2004/0213705 A1 by Blencoe et al. describes aprocess for sequestering carbon dioxide from a gas stream viadissolution of a metal silicate with a caustic material to produce ametal hydroxide and subsequently contacting said metal hydroxide withthe carbon dioxide to produce a metal carbonate. No teaching is given inrelation to other non-caustic routes to carbonation, nor to other meansof activation of the metal silicate. It is noted that methods thatrequire strong caustic or acidic dissolution of the metal are expectednot to be viable industrial processes for large-scale carbon dioxidesequestration due to the high energy and raw material requirements toprovide such strong caustic or acid solvents in large quantities.

US Patent Application No. 2001/0022952 A1 by Rau and Caldeira describesa process for sequestering carbon dioxide from a gas stream by hydratingthe carbon dioxide to form carbonic acid and reacting the resultingcarbonic acid with a carbonate. This process is quite different to thatof the current invention.

US Patent application No. 2004/0219090 A1 by Dziedic et al. describes aprocess for removing carbon dioxide from a gaseous stream by diffusingcarbon dioxide into water, adding a catalyst to accelerate theconversion of the carbon dioxide to carbonic acid and adding a mineralion to form a precipitate of a salt of the carbonic acid. This processis also quite different to that of the current invention, although maybe advantageously used in conjunction with the current inventionparticularly for the sequestration of carbon dioxide directly from theatmosphere.

Hitherto no research or prior art has described a process that iscapable of successfully providing for sequestering carbon dioxide byreaction with alkali metal or alkaline earth metal silicates to formalkali metal or alkaline earth metal carbonates in an integrated waythat is both energy efficient and technically and economically viablefor industrial operations. It would be highly advantageous to providesuch a process. All published work on aqueous routes has concluded thatthe energy penalty for activation and dehydroxylation of alkali metal oralkaline earth metal silicate minerals such as serpentine rule out thisapproach for viable industrial carbonation processes. It has now beendiscovered however that an integrated process with direct thermalactivation via combustion combined with suspension in suitable solventsand the application of selected process routes renders the overallmineral carbonation process, especially for feedstocks such asserpentine, far more energy efficient and economically viable than hasheretofore been envisaged. This new approach renders mineral carbonationusing serpentine a viable industrial process for the first time.Economic viability depends on achieving a relatively low overall costper tonne of carbon dioxide sequestered, preferably costs that would bebelow the market prices of carbon dioxide under regimes of carbon taxesor carbon emissions trading or permits. The present invention providessuch a process.

Accordingly, the present invention provides a process for thesolidification of carbon dioxide of by reaction of carbon dioxide withan alkali metal or alkaline earth metal silicate feedstock to form acorresponding alkali metal or alkaline earth metal carbonate, whichprocess comprises direct thermal activation of the alkali metal oralkaline earth metal silicate feedstock by combustion of fuel to producean activated feedstock, suspending the activated feedstock in a solventslurry and contacting the activated feedstock with carbon dioxide toconvert the carbon dioxide to form an alkali metal or alkaline earthmetal carbonate.

The process of the present invention advantageously provides a means forsequestering carbon dioxide by conversion of carbon dioxide into stablealkali metal or alkaline earth metal carbonates. The process therebyprovides a means for reducing the amount of carbon dioxide released tothe atmosphere.

An important aspect of the present invention involves direct thermalactivation of the alkaline or alkaline earth metal silicate feedstockfor reaction with carbon dioxide. Activation is achieved by combustionof a fuel with the heat released being applied directly to thefeedstock. In the context of the present invention the use ofelectricity to provide the heat for activation of the feedstock, forexample, using an electric furnace, would involve indirect thermalactivation since the heat of combustion of fuel (to generateelectricity) is not being applied directly to heat the feedstock. Thisis energetically disadvantageous.

In accordance with the present invention the fuel used to achieve directthermal activation of the feedstock is invariably a carbonaceous orhydrocarbonaceous fuel, such as coal, oil or natural gas.

Thermal activation of the feedstock may take place in any suitableheating vessel. This will usually take the form of a kiln, furnace orsimilar combustion chamber or heater. The feedstock may be contactedwith the combustion gases from the fuel or may be heated via radiation,conduction or convection from the fuel combustion chamber. The heatingvessel may be designed to provide turbulent or dispersive or attritiveconditions to assist in achieving the dehydroxylation of the feedstockessential for activation. Thus, the reaction vessel may be designed torotate and/or agitate the feedstock during heating thereof to assist indehydroxylation (activation).

The feedstock is typically transported as a ground solid through theheating vessel. In one embodiment the heating vessel may be of verticalshaft design comprising one or more substantially vertical chambers andwherein the feedstock is charged and flows counter-currently to gasesproduced by the combustion of the fuel. Alternatively, the solidfeedstock may be transported through the heating vessel in fluid mediain pipes or vessels, such fluids being either gases or liquids.

Reaction of carbon dioxide with activated feedstock is exothermic. In anembodiment of the invention the activated feedstock is pre-heated priorto direct thermal activation using heat liberated by the exothermic(downstream/subsequent) reaction. In this embodiment a series of heatexchanges may be used to convey heat to the feedstock. Additionally, oralternatively, pre-heating may utilise low grade or waste heat from anassociated carbonaceous or hydrocarbonaceous combustion, gasificationand/or reforming process. Pre-heating of the silicate feedstock in thisway will make the process of the invention more energeticallyeconomical.

Pre-heating may utilise a series of heating vessels successivelyutilising the exothermic heat of the subsequent carbonation reactionand/or low grade or waste heat from an associated carbonaceous orhydrocarbonaceous fuel combustion, gasification or reforming process

Activation of the silicate feedstock typically involves raising andfinally maintaining the temperature of said feedstock to a temperatureof from about 580 and 800 degrees Celsius. While the use of heat fromthe exothermic heat of the carbonation reaction and/or low grade orwaste heat from an associated hydrocarbon fuel combustion, gasificationor reforming process for pre-heating the alkali metal or alkaline earthmetal containing streams may make this process more energy and costefficient, these steps are not absolutely essential. All of the energyrequired to achieve activation energy may be supplied by an efficientheating vessel. This process, particularly with agitation applied in thecombustion vessel or heater, has now been found to provide a moreenergy-efficient and hence industrially viable process for carbondioxide sequestration via ex-situ mineral carbonation.

Preferably, the activated feedstock suspended in a solvent slurry issubsequently contacted with supercritical, liquefied or high-pressuregaseous carbon dioxide to substantially convert the carbon dioxide toalkali metal or alkaline earth metal carbonates. The term high-pressurein the context of this disclosure refers to pressures in excess of 5bar, more preferably in excess of 20 bar.

The most suitable fuel for combustion may be the same fuel used in theassociated hydrocarbon fuel combustion, gasification or reformingprocess, carbon dioxide emissions from which are to be subject to themineral carbonation process of this invention. In general, due to thehigh masses of mineral required to sequester carbon dioxide emissions, amineral carbonation plant should desirably be sited close to the alkalimetal or alkaline earth metal silicate mine or quarry. Where the site ofthe mineral carbonation plant is remote from the associated carbonaceousor hydrocarbonaceous fuel combustion, gasification or reforming processplant, the carbon dioxide has to be transported to the mineralcarbonation plant via pipelines or the like and the option of using lowgrade or waste heat from the said associated plant is not available. Ingeneral, larger masses of mineral are required than the correspondingmasses of carbonaceous or hydrocarbonaceous fuel used in the associatedcombustion, gasification or reforming process plant whose carbon dioxideemissions are subject to the mineral carbonation process. This makes itmore favourable to situate such a combustion, gasification or reformingprocess plant in close proximity to the alkali metal or alkaline earthmetal silicate mine or quarry itself. Transport of the carbonaceous orhydrocarbonaceous fuel to the combined combustion and carbonation plantsite is thus less costly in an overall sense and is the preferredoption.

The associated hydrocarbon fuel combustion, gasification or reformingprocess may comprise or form part of a coal, oil or gas-firedelectricity generation plant, ammonia or other chemical manufacturingplant, Portland cement plant or the like. Most commonly the saidassociated plant will be an electricity generation plant, most commonlya coal-fired electricity generation plant.

In a particular embodiment of this invention the carbonaceous orhydrocarbonaceous fuel used in the combustion, gasification, reformingor electricity generation plant comprises at least 20%, preferably20-100%, of fuel derived from renewable biomass, thus providing anoverall process for the net removal of carbon dioxide from theatmosphere while providing thermal or electrical energy or hydrogen forutilisation in downstream energetic processes.

Similarly, the carbonaceous or hydrocarbonaceous fuel that is combustedto provide thermal energy to the alkali metal or alkaline earth metalsilicate feedstock may advantageously comprise at least 20%, preferably20-100%, of fuel derived from renewable biomass. This provides a processof thermal activation that does not produce excessive additional carbondioxide from the mineral carbonation process itself. Renewable biomassfuel is particularly suited to this thermal activation process sincetemperatures below about 800 degrees Celsius are required.Advantageously, oxygen or oxygen enriched air may be fed into theheating vessel to provide a flue stream made up largely of carbondioxide and water that may be fed back into the mineral carbonationplant for sequestration of the carbon dioxide.

The most preferable alkali metal or alkaline earth metal silicatefeedstock is serpentine or one of its polymorphs. However, feedstocksdrawn from the group comprising serpentine and any of its polymorphsantigorite, lizardite or chrysotile, olivine, brucite, dunite,peridotite, forsterite, wollastonite, talc, harzburgite, and mixturesthereof, may be used in the present invention.

In general the feedstock will be subjected to comminution by crushingand/or grinding subsequent to its extraction. Comminution to the finaldesired particle size distribution for the carbonation reaction may bedone either before or after the direct thermal heating step. The saidfinal desired particle size distribution for the carbonation reaction isabout 75 microns or less. It may be advantageous to perform comminutionto a size of about 200 microns or less prior to said direct combustionheating followed by subsequent further comminution to the said finaldesired particle size distribution for the carbonation reaction. Suchsubsequent grinding may advantageously be done in a wet grinding processwith the activated feedstock mixed with the solvent slurry prior to themineral carbonation step.

The most preferable process involves pre-heating of the silicatefeedstock using one or more heating vessels utilising heat recoveredfrom the exothermic carbonation reaction, which will generally be attemperatures below 200 degrees Celsius, more commonly below about 150degrees Celsius. Further heating may be achieved utilising low-gradeheat recovered from an associated hydrocarbon fuel combustion,gasification or reforming plant, as described. Finally, and essentiallyfor this process, the pre-heated silicate feedstock is heated in asuitable heating vessel to its required activation temperature ofbetween about 580 and 800 degrees Celsius. These temperatures areconsiderably lower than those typically employed in calciningoperations, making the use of such a heating vessel more energyefficient and allowing lower cost refractory materials to be used in itsconstruction.

Suitable heating vessels include rotary kilns and shaft or tower kilns.The most energy efficient designs, such as multistage counter-currentregenerative shaft or tower kilns, are preferred. It has been found thatthe most energy efficient designs used in other industrial applicationssuch as the calcining of lime are particularly advantageous whensuitably modified for application in the current invention. Such designsinclude fluidised bed kilns or alternatively rotary kilns with axialcombustion chambers and multiple co-axial calcining chambers. The lowertemperatures required for the activation of the silicate feedstock inthe current application as compared to conventional calcining enableconsiderable reductions in the design requirements of such kilns. Thisenables both capital and operational cost savings to be achieved inemploying this type of unit.

Agitation of the mineral feedstock in the heating vessel is beneficialto the process of activation of the feedstock and may advantageously beemployed in the heating vessel. Such agitation may be applied viarotation in rotary kilns, preferably in the presence of some additionalgrinding and/or agitation media such as steel balls. Alternatively, someagitation may be obtained via counter-current gas flow in shaft or towerkilns or fluidised bed furnaces, again preferably in the presence ofsome additional grinding and/or agitation media.

Transport of the mineral feedstock through pipes or chambers in theheating vessel may alternatively be achieved by two-phase fluid/solidflow, said fluids comprising either gases or liquids. For the case ofgas/solid flow, the carrier gas provides agitation and efficient heattransfer which may be enhanced by high gas flow rates during transportof said mineral feedstock through the heating vessel.

It may be advantageous to transport the mineral feedstock as slurrysuspended in a liquid carrier as it passes through the heating vessel.In this regard, aqueous media are preferred, with the most preferablemedia comprising those used in the carbonation step; namely weaklyacidic aqueous or mixed aqueous and/or saline or other liquid solvents.As for the carbonation reaction, the solvents may be chosen from any ofwater, weak acids such as those known in the prior art for exampleacetic acid, oxalic acid, ascorbic acid, phthalic acid, orthophosphoricacid, citric acid, formic acid or salt solutions of such weak acids,saline solutions, aqueous saline and sodium bicarbonate solutions,potassium bicarbonate solutions, mixed aqueous and alcohol solutionssuch as aqueous ethanol or methanol solutions, mixed aqueous and glycolsolutions, mixed aqueous and glycerol solutions, or any combinationthereof.

It is preferable that the ratio of liquids to solids in the directthermal activation stage be kept low, and usually lower than thatemployed in the carbonation step in order to reduce thermal energyrequirements in raising the slurry to its desired temperature range ofbetween about 580 and 800 degrees Celsius for mineral activation. Underthese conditions the liquids will generally be superheated. The presenceof the liquid carrier assists in the dehydroxylation of the silicatefeedstock, by providing efficient heat transfer, turbulent flow and somedissolution of the alkali metal or alkaline earth metal and by assistingdisruption of silica layers.

In the embodiments for transport of the feedstock via fluid carriers,said carriers comprising either gases or liquids, the thermal energysupplied to the heating vessel may be reduced via recycling of thecarrier fluid through said heating vessel. The solid mineral feedstockmay be substantially separated from the carrier fluid after exiting theheating vessel and then recycled to carry more mineral feedstock throughthe heating vessel, thus maintaining most of the thermal energy of theheated fluid. Substantial solid/fluid separation may be achieved bywell-known process methods such as gravity separation, centrifugalseparation or filtration.

It will be appreciated that the use of process units such as kilns,furnaces or other heating vessels, comminution processes and reactionvessels referred to in this specification is not limited to anyparticular number of such vessels. Plural such units may be employed,either in series or parallel, in order to provide the required processthroughput for any particular mineral carbonation facility. For example,in order to solidify and sequester about 15 million tonnes of carbondioxide produced annually by a gigawatt-scale coal-fired electricitygeneration plant, about 40 million tonnes of serpentine would need to beprocessed annually. This requires a facility processing in excess of 100kilotonnes of serpentine per day or in excess of 4500 tonnes per hour.Multiple large parallel processing units are required to meet suchthroughput.

Preferably, after the direct thermal activation step the activatedfeedstocks are suspended in weakly acidic aqueous or mixed aqueousand/or saline or other solvents prior to the carbonation step.Advantageously, the aqueous solvent system described by O'Connor et al.comprising an aqueous saline solution with sodium bicarbonate may beemployed. Other suitable solvents that have been identified by workersin this field include potassium bicarbonate solutions.

Preferably, the said activated feedstocks suspended in the solvents arecontacted with supercritical, liquefied or high-pressure gaseous carbondioxide in highly turbulent or rapidly dispersive or attritive reactionvessels to substantially convert the carbon dioxide to carbonates.Preferably pressures in the range 10-200 bar, more preferably 50-160 barand temperatures in the range 10-250 degrees Celsius, more preferably10-175 degrees Celsius are employed in the reaction vessels.

Suitable reaction vessels may comprise high-pressure agitated vessels,pipeline reactors or the like, or more preferably, high velocityreaction vessels to promote turbulence, rapid mixing and attrition ofthe said activated feedstocks. Fluidised bed reactors such as describedby Park and Fan, particularly with the addition of grinding media, maybe advantageously employed. Furthermore, the process as described byPark and Fan of elevating the pH in said reaction vessel to facilitateprecipitation of the carbonates may be advantageously applied.

According to another aspect of the invention, there is provided aprocess for long-term sequestration of carbon dioxide from theatmosphere into solid alkali metal or alkaline earth metal carbonateswhereby, after mining of feedstock that comprise alkali metal oralkaline earth metal silicates, comminution and direct thermalactivation of said feedstock, the activated feedstock are suspended in asolvent slurry comprising solvents that are miscible with liquid carbondioxide and/or capable of increased dissolution of carbon dioxide andare contacted with carbon dioxide in reaction vessels to substantiallyconvert the carbon dioxide to alkali metal or alkaline earth metalcarbonates.

The solvents may be chosen from any of water, weak acids such as thoseknown in the prior art for example acetic acid, oxalic acid, ascorbicacid, phthalic acid, orthophosphoric acid, citric acid, formic acid orsalt solutions of such weak acids, saline solutions, aqueous saline andsodium bicarbonate solutions, potassium bicarbonate solutions, mixedaqueous and alcohol solutions such as aqueous ethanol or methanolsolutions, mixed aqueous and glycol solutions, mixed aqueous andglycerol solutions or any combinations thereof. The final choice ofsolvent will be dependent on the need to provide suitable reactionconditions and buffering as taught by O'Connor et al. for thecarbonation reactions as well as to provide suitable miscibility withthe high-pressure, supercritical or liquefied carbon dioxide in thecarbonation reaction vessel.

Another application of this invention may be in the sequestration ofcarbon dioxide drawn from dilute streams or directly from the atmospherein order to reduce the carbon dioxide concentration in the atmosphere tomitigate the effects of global warming and climate change. In thisregard, Lackner et al. presented a conceptual outline of such a processshowing that from physical considerations it is feasible to constructstructures to absorb substantial quantities of carbon dioxide from theair. They do not present any detailed chemical process for theabsorption and solidification of the carbon dioxide except to name theuse of calcium oxide as a possible substrate. It will be apparent tothose skilled in the art that the processes such as those disclosed inthe current invention may be adapted and used for such absorption andsolidification of carbon dioxide from the atmosphere. Key aspects andthe associated process improvements and applications disclosed hereinmay be employed in such processes. In particular, the use of the thermalactivation processes via combustion disclosed herein and the solventprocesses as described herein as well as the other various processimprovements and applications described herein may be employed in suchcapture of carbon dioxide from the atmosphere. Atmospheric carbondioxide may be concentrated prior to reaction, for example via suchcapture and concentration processes described by Lackner et al. or maybe sequestered in dilute form, including direct reaction withatmospheric carbon dioxide. In the latter case, the sequestration mayproceed more slowly than in high-pressure reaction vessels, neverthelessusing suitably activated alkali or alkali earth metal silicates such asserpentine and/or suitably selected slurry solvents to convert thecarbon dioxide to carbonates. Systems of open vessels, fields, slurrydams, absorption towers, aerated stockpiles or heap leach arrangementscontaining the activated serpentine or other alkali or alkali-earthmetal silicate mixed with such solvents may be employed in thisapplication Such vessels, fields, slurry dams, absorption towers oraerated stockpiles or heap leach arrangements may be designed tooptimally expose the activated mineral to carbon dioxide, preferablydissolved as carbonic acid in aqueous media, via systems of sprays,atomizers, or channels. The reacted mineral, in the form of carbonates,should be periodically removed to allow exposure of unreacted mineral tothe carbon dioxide or carbonic acid/aqueous flows. In the case ofstockpiles for example, reacted layers may be periodically scraped offthe exposed surfaces of said stockpiles. The removed material comprisingcarbonates may then be transported for disposal, such disposal beingadvantageously back in mined-out areas of the mineral feedstock mine orquarry.

It may be desirable to enhance the dissolution of atmospheric carbondioxide into carbonic acid in aqueous media prior to reaction with theactivated mineral. Such enhancement may be obtained via means known inthe prior art, for example via the addition of enzyme catalysts such ascarbonic anhydrase to the aqueous media as described by Dziedzic et al.Preferably, the enzyme catalyst would be recycled.

Various embodiments of a method for long-term sequestration of carbondioxide into solid alkali metal or alkaline earth metal carbonates inaccordance with the present invention will now be described, by way ofexample only, with reference to the accompanying drawings.

FIG. 1 illustrates a generalised flow diagram of the invention. It showsa process for activation of an alkali earth metal silicate ore, in thiscase largely serpentine ore, using the methodology of this invention. Itshows a mine or quarry (1) where the ore is extracted, an associatedcombustion, gasification, reforming or electricity generation plant (2)whose carbon dioxide emissions are to be sequestered and a stream (3)containing the said carbon dioxide entering a mineral carbonation plant(5) designed according to the methodology of this invention. Theserpentine ore is crushed and ground in comminution circuits (6) to aparticle size of less than 75 microns and fed into a series of heatexchangers for activation. The first optional heat exchanger (7)utilises heat drawn from maintaining the carbonation reactor (8) at atemperature of 120-150 degrees Celsius drawing heat from the exothermiccarbonation reaction within the said reactor. The second optional heatexchanger (9) utilises low grade heat drawn from an available low gradeheat source (4) in the associated combustion, gasification, reforming orelectricity generation plant (2), in this case further raising thetemperature of the serpentine ore to around 300 degrees Celsius. Thefinal and essential heating vessel (10) comprises a hydrocarbonaceousfuel-fired furnace, kiln or similar combustion chamber to provide directthermal activation of the ore raising its temperature to around 580 to800 degrees Celsius. The activated ore is mixed with a solvent (11)prior to entering the carbonation reactor vessel (8). The carbonationreaction (8) vessel may advantageously utilise agitation and attrition,either via mechanical means or flow-induced. The solvents (11) areaqueous mixtures of water with weak acids, and/or salts and/or sodiumbicarbonate. The carbon dioxide-containing stream (3) is compressed viacompressor (12) to a liquid form or to a pressure in excess of 150 barprior to entering said carbonation reactor vessel (8). The solidcarbonate and silica residues (13) are withdrawn for final disposal backto the mine or quarry (1) and the recovered solvents (14) are reused inthe process.

The process illustrated in FIG. 1 has been demonstrated to beeconomically viable for the permanent solidification of 14.1 Mt perannum of carbon dioxide emissions from a standard conventionalpulverised fuel electricity generation plant in Australia. The powerstation has four 660 MW generators that export about 15500 GWh per annumto the electricity grid and consumes 6.4 Mt per annum of black coal. Theprocess shown in FIG. 1 achieves close to 100% permanent carbon dioxidesequestration with about 41 Mt per annum of serpentine and additionalcoal consumption of 0.9 Mt per annum in the fuel-fired furnaces thatactivate the serpentine. Delivered electricity from the electricitygeneration plant would be reduced to 96.6% of the original supplywithout sequestration due to the requirement to supply electricity forthe comminution of the serpentine. The process will avoid 14.1 Mt carbondioxide at a cost of about Australian dollars A$22 per tonne of carbondioxide. In terms of electricity generation costs, the penalty of nearly100% carbon dioxide sequestration using this process has beendemonstrated to be about 2.1 c/kWh.

FIG. 2 illustrates another generalised flow diagram of the inventionsimilar to FIG. 1. All components are identical to those illustrated inFIG. 1 except for the addition of a solvent stream (15) to the alkaliearth metal silicate ore prior to thermal activation in order totransport said ore through the thermal activation heat exchangers.

FIG. 3 illustrates another generalised flow diagram of the inventionsimilar to FIG. 1. All components are again identical to thoseillustrated in FIG. 1 except for the addition of a gas stream (15), inthis example compressed air, to the alkali earth metal silicate oreprior to thermal activation in order to transport said ore through thethermal activation heat exchangers.

FIG. 4 illustrates another generalised flow diagram of the invention. Itshows a process for activation of an alkali earth metal silicate ore, inthis case largely serpentine ore, using the methodology of thisinvention. It shows a mine or quarry (1) where the ore is extracted, anassociated combustion, gasification, reforming or electricity generationplant (2) whose carbon dioxide emissions are to be sequestered and astream (3) containing the said carbon dioxide entering a mineralcarbonation plant (5) designed according to the methodology of thisinvention. The serpentine ore is crushed and ground in comminutioncircuits (6) to a particle size of less than 200 microns and fed into aseries of heat exchangers for activation. The optional first heatexchanger (7) utilises heat drawn from maintaining the carbonationreactor (8) at a temperature of 120-150 degrees Celsius drawing heatfrom the exothermic carbonation reaction within the said reactor. Theoptional second heat exchanger (9) utilises low grade heat drawn from anavailable low grade heat source (4) in the associated combustion,gasification, reforming or electricity generation plant (2), in thiscase further raising the temperature of the serpentine ore to around 300degrees Celsius. The final and essential heat exchanger (10) comprises ahydrocarbonaceous fuel-fired furnace, kiln or similar combustion chamberto provide direct thermal activation of the ore raising its temperatureto around 580 to 800 degrees Celsius. The heating vessel (10) is atwo-stage counter-current tower furnace to improve thermal efficiency.Optionally, it may utilise a fluidised bed of the mineral ore. Theactivated ore is mixed with an aqueous solvent stream (11) containing aweak acid and subjected to further comminution in a wet-milling process(12) to a particle size of less than 75 microns before being mixed withadditional solvents (13) comprising weak acids, and/or salts and/orsodium bicarbonate and optionally alcohol and/or glycol or glycerolsolvent to render carbon dioxide more miscible prior to entering thecarbonation reactor vessel (8). The carbon dioxide-containing stream (3)is mixed with carbon dioxide from the hydrocarbonaceous fuel-firedfurnace, kiln (10) and compressed via compressor (14) to a liquid formor to a pressure in excess of 150 bar prior to entering said carbonationreactor vessel (8). The carbonation reaction (8) vessel mayadvantageously utilise agitation and attrition, either via mechanicalmeans or flow-induced. The solid carbonate and silica residues (15) arewithdrawn for final disposal back to the mine or quarry (1) and therecovered solvents (16) are reused in the process.

FIG. 5 illustrates another generalised flow diagram of the invention. Inthis case a similar process to that described in FIG. 2 applies andunless otherwise state here comprises components labelled as for FIG. 2.In this example the associated combustion, gasification, reforming orelectricity generation plant (2) utilising between 20 and 100% ofrenewable biomass (17) yielding an overall process for the net removalof carbon dioxide from the atmosphere. In this example, the heatingvessel (10) comprises a fuel-fired furnace, kiln or similar combustionchamber that similarly combusts hydrocarbonaceous fuel derived largelyfrom renewable biomass (18) to provide direct thermal activation of theore raising its temperature to around 580 to 800 degrees Celsius and isoperated with an oxygen-rich feed stream (19) to provide a flue stream(20) largely comprising carbon dioxide and water vapour that is fed backinto the mineral carbonation plant (5).

FIG. 6 illustrates another generalised flow diagram of the inventionsimilar to that described in FIG. 1 and unless otherwise state herecomprises components labelled as for FIG. 1. In this example the heatingvessel (10) comprises a rotary kiln with grinding media (15) thatprovides mechanical agitation and attrition while simultaneouslyproviding thermal activation of the ore by raising its temperature toaround 580 to 800 degrees Celsius by combustion of hydrocarbonaceousfuel. This heating vessel (10) may optionally and advantageously besupplied by fuel comprising between 20-100% of renewable biomass (16)and may also optionally be operated with an oxygen-rich feed stream (17)to provide a flue stream (18) largely comprising carbon dioxide andwater vapour that is fed back into the mineral carbonation plant (5).

FIG. 7 illustrates another generalised flow diagram of the invention. Inthis example the process is similar to that shown in FIG. 2 and alsoincorporates some of the features shown in FIG. 4. Unless otherwisestated here the components are labelled as for FIG. 2 except that inthis example the heating vessel (10) comprises a rotary kiln withgrinding media (15) that provides mechanical agitation and attritionwhile simultaneously providing thermal activation of the ore by raisingits temperature to around 580 to 800 degrees Celsius by combustion ofhydrocarbonaceous fuel. This heating vessel (10) may optionally andadvantageously be supplied by fuel comprising between 20-100% ofrenewable biomass (17) and may also optionally be operated with anoxygen-rich feed stream (18) to provide a flue stream (19) largelycomprising carbon dioxide and water vapour that is fed back into themineral carbonation plant (5). The activated ore is mixed with anaqueous solvent stream (11) containing a weak acid and subjected tofurther comminution in a wet-milling process (12) to a particle size ofless than 75 microns before being mixed with additional solvents (13)including alcohol and/or glycol or glycerol solvent to render carbondioxide more miscible prior to entering the carbonation reactor vessel(8). The carbonation reaction (8) vessel may advantageously utiliseagitation and attrition, either via mechanical means or flow-induced.The carbon dioxide-containing stream (3) is mixed with carbon dioxidefrom the hydrocarbonaceous fuel-fired furnace, kiln (10) and compressedvia compressor (14) to a liquid form or to a pressure in excess of 150bar prior to entering said carbonation reactor vessel (8). The solidcarbonate and silica residues (15) are withdrawn for final disposal backto the mine or quarry (1) and the recovered solvents (16) are reused inthe process.

FIG. 8 illustrates another flow diagram of a particular embodiment ofthe invention. In this example the mineral carbonation plant (5) issimilar to that shown in FIG. 5 however in this case it is used tosequester carbon dioxide from the atmosphere. The carbon dioxide isdrawn from the atmosphere in a generic capture plant (2) thatconcentrates the carbon dioxide (4) and feeds it in a stream (3) to themineral carbonation plant (5) whose details are similar to those of FIG.5 and unless specified otherwise comprises components labelled as forFIG. 5.

FIG. 9 illustrates another flow diagram of a particular embodiment ofthe invention. It shows a process for activation of an alkali earthmetal silicate ore, in this case largely serpentine ore, using themethodology of this invention and the use of such activated ore tosequester carbon dioxide from dilute streams or under ambientconditions. It shows a mine or quarry (1) where the ore is extracted andthe ore entering a mineral carbonation preparation plant (2) designedaccording to the methodology of this invention. The serpentine ore iscrushed and ground in comminution circuits (3) to a particle size ofless than 200 microns and fed into a heating vessel (4) comprising ahydrocarbonaceous fuel-fired furnace, kiln or similar combustion chamberto provide direct thermal activation of the ore raising its temperatureto around 580 to 800 degrees Celsius. The heating vessel shown here is arotary kiln containing internal grinding media (5), however it mayoptionally be a multi-stage counter-current tower furnace to improvethermal efficiency. Optionally, it may utilise a fluidised bed of themineral ore. The activated ore is mixed with an aqueous solvent stream(7) containing mixtures of water with weak acids, and/or salts and/orsodium bicarbonate and subjected to further comminution in a wet-millingprocess (8) to a particle size of less than 75 microns. The activatedore is then exposed to dilute carbon dioxide streams in a carbonationzone (9) to convert the carbon dioxide to a mineral carbonate. Suchcarbonate may be periodically removed from the carbonation zone toexpose unreacted activated ore to more carbon dioxide. The carbonationzone may comprise specifically designed vessels to perform such exposureto carbon dioxide and removal of reacted carbonate or may alternativelycomprise large open fields, slurry dams, stockpiles or similar aeratedstructures or heap leach arrangements to expose the activated mineral tothe carbon dioxide. Some addition of additional solvents or water may berequired in this carbonation zone. The reacted carbonates and residuesilicates (10) may be returned to the mine or quarry (1) for disposal.The carbonation zone (9) may itself be situated within the mine orquarry (1).

FIG. 10 illustrates another generalised flow diagram of the inventionsimilar to FIG. 9. All components are identical to those illustrated inFIG. 9 except for the addition of a system of sprays or flowdistributors (12) over the vessels, open fields, slurry dams, stockpilesor similar aerated structures or heap leach arrangements that sprayaqueous solutions (11) that may contain catalytic enzymes such ascarbonic anhydrase to accelerate formation of carbonic acid. Thesestreams are recycled (13).

It will be apparent to those skilled in the art that variousmodifications, omissions or additions may be made without departing fromthe scope of the invention which is not limited to the specificembodiments and examples described herein. It is to be understood thatthe invention includes all such variations and modifications that fallwithin the spirit and scope. The invention also includes all of thesteps, features, compositions and compounds referred to or indicated inthis specification, individually or collectively, and any and allcombinations of any two or more of said steps or features.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the word “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated integer or step or group of integers or steps but not theexclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and shouldnot be taken as, an acknowledgement or any form of suggestion that priorart forms part of the common general knowledge of the countries in whichthis application is filed.

The claims defining the invention are as follows:
 1. A process for thesolidification of carbon dioxide by reaction of carbon dioxide with anactivated alkali metal silicate feedstock or an activated alkaline earthmetal silicate feedstock to form a corresponding alkali metal carbonateor alkaline earth metal carbonate, which process comprises directthermal activation of an alkali metal silicate feedstock or alkalineearth metal silicate feedstock by combustion of fuel in a fuel firedfurnace with the heat released by said combustion being directly appliedto the alkali metal silicate feedstock or alkaline earth metal silicatefeedstock, to produce an activated feedstock, then suspending theactivated feedstock in a solvent slurry and reacting the activatedfeedstock with carbon dioxide to form the alkali metal carbonate oralkaline earth metal carbonate.
 2. A process according to claim 1,wherein the alkali metal silicate feedstock or alkaline earth metalsilicate feedstock is pre-heated prior to direct thermal activation bycombustion of the fuel using heat liberated from the reaction of carbondioxide with the activated feedstock and/or low grade or waste heatdrawn from an associated carbonaceous or hydrocarbonaceous fuelcombustion, gasification or reforming process.
 3. A process according toclaim 1, wherein direct thermal activation of the alkali metal silicatefeedstock or alkaline earth metal silicate feedstock takes place byraising and maintaining the temperature of said feedstock to atemperature of from about 580° C. to 800° C.
 4. A process according toclaim 1, wherein the alkali metal silicate feedstock or alkaline earthmetal silicate feedstock is heated in a heating vessel that is designedto rotate and/or agitate the feedstock during heating thereof.
 5. Aprocess according to claim 1, wherein the alkali metal silicatefeedstock or alkaline earth metal silicate feedstock is heated in aheating vessel of a vertical shaft design comprising one or moresubstantially vertical chambers and wherein the feedstock is charged andflows counter-currently to gases produced by the combustion of the fuel.6. A process according to claim 1, wherein the alkali metal silicatefeedstock or alkaline earth metal silicate feedstock is heated viaradiation, conduction or convection from a chamber in which combustionof the fuel takes place.
 7. A process according to claim 1, wherein thealkali metal silicate feedstock or alkaline earth metal silicatefeedstock is transported in pipes or vessels through the furnace influid media, such fluids being either gases or liquids.
 8. A processaccording to claim 1, wherein the alkali metal silicate feedstock oralkaline earth metal silicate feedstock is subjected to comminution toreduce an average particle size of said feedstock to less than about 200microns.
 9. A process according to claim 1, wherein the activatedfeedstock is suspended in a weakly acidic aqueous or mixed aqueousand/or saline or other solvent miscible with carbon dioxide beforeand/or after the direct thermal activation.
 10. A process according toclaim 9, where the solvent is chosen from any one or more of water, weakacids or salt solutions of weak acids, saline solutions, aqueous salineand sodium bicarbonate solutions, potassium bicarbonate solutions, mixedaqueous and alcohol solutions.
 11. A process according to claim 10,wherein the weak acid is selected from the group consisting of aceticacid, oxalic acid, ascorbic acid, phthalic acid, orthophosphoric acid,citric acid, and formic acid.
 12. A process according to claim 10,wherein the mixed aqueous and alcohol solution is selected from thegroup consisting of aqueous ethanol or methanol solutions, mixed aqueousand glycol solutions, and mixed aqueous and glycerol solutions.
 13. Aprocess according to claim 1, wherein direct thermal activation of thealkali metal silicate feedstock or alkaline earth metal silicatefeedstock by combustion of fuel occurs in a fluidised bed furnace orfluidised bed kiln.
 14. A process according to claim 1, wherein theactivated feedstock is mixed with an aqueous solvent stream containing aweak acid and subjected to further comminution after the direct thermalactivation step in a wet-milling process to a particle size of less than75 microns.
 15. A process according to claim 1, wherein the activatedfeedstock is reacted with supercritical, liquefied or high-pressuregaseous carbon dioxide to substantially convert the carbon dioxide toalkali metal or alkaline earth metal carbonates.
 16. A process accordingto claim 1, where the carbon dioxide and activated feedstock are reactedin a reaction vessel that is designed to provide highly turbulent orrapidly dispersive or attritive conditions to rapidly and substantiallyconvert the carbon dioxide to alkali metal or alkaline earth metalcarbonates.
 17. A process according to claim 1, wherein the carbondioxide is derived from emissions from a carbonaceous fuel combustionprocess, hydrocarbonaceous fuel combustion process, or hydrocarbongasification process or reforming processes.
 18. A process according toclaim 17, wherein the carbon dioxide is derived from flue emissions fromcoal, oil or natural gas-fired electricity generation.
 19. A processaccording to claim 1, wherein the carbon dioxide gas is derived from anammonia manufacturing plant.
 20. A process according to claim 1, whereinthe carbon dioxide gas is derived from a Portland cement manufacturingplant.
 21. A process according to claim 1, wherein the carbon dioxide isderived from the oxidation of at least 20% of fuel derived fromrenewable biomass.
 22. A process according to claim 1, wherein the fuelcomprises at least 20% of fuel derived from renewable biomass.
 23. Aprocess according to claim 1, wherein the alkali metal silicatefeedstock or alkaline earth metal silicate feedstock comprisesserpentine or a polymorph thereof, antigorite, lizardite, chrysotile,olivine forsterite, brucite, dunite, peridotite, wollastonite, talc,harzburgite, or a mixture of any two or more thereof.
 24. A processaccording to claim 1, wherein the combustion of the fuel is achievedwith the addition of oxygen-enriched streams to generate carbon dioxideand water vapour for ease of subsequent solidification into carbonates.25. A process according to claim 1, wherein the alkali metal silicatefeedstock or alkaline earth metal silicate feedstock is crushed incrushers and ground in mills in comminution circuits that drawelectrical energy produced from an associated electricity generationplant.
 26. A process according to claim 1, wherein the carbon dioxide isdrawn directly from the atmosphere.
 27. A process according to claim 26,wherein the reaction of carbon dioxide with the activated feedstocktakes place in systems of open fields, slurry dams, stockpiles or heapleach arrangements containing the activated feedstock.
 28. A processaccording to claim 27, wherein sprays, atomizers or channels are used todistribute aqueous streams through the systems of open fields, slurrydams, stockpiles or heap leach arrangements containing the activatedfeedstock.
 29. A process according to claim 28, wherein carbonatesproduced by reaction of the carbon dioxide and activated feedstock areperiodically removed from the systems.
 30. A process according to claim28, wherein an enzyme catalyst is added to the aqueous streams toaccelerate the formation of carbonic acid.